This invention relates to rubber concentrates and polymer blends based thereon. Rubber concentrates are often blended or copolymerized with polymers to increase the impact strength of the resulting polymer composition. ABS and impact polystyrene are examples of such polymer compositions.
One limit on the continued addition of rubber to such compositions is that the viscosity of the composition increases excessively. A high viscosity polymer composition is too difficult to process.
Additionally, increasing rubber content also becomes marginally less effective because of particle size limitations. In an ABS, for example, the rubber particles should have a particle size of approximately 0.7 microns in order to maximize impact strength. As one adds more rubber to the ABS composition, it becomes more difficult to disperse the rubber in particles that small in size.
Merely using a lower molecular weight rubber does not solve these problems. While it does improve processability and make it easier to disperse the rubber into smaller particles, the resulting rubber reinforced polymer composition is not tough enough.
As a result, the typical ABS polymer composition has a rubber content of only about 15%. The maximum is usually about 20%.
U.S. Pat. No. 4,233,418 to Lingier et al., issued Nov. 11, 1980, does recognize the desirability of getting more rubber into an ABS plastic. It achieves this result by suspension polymerization, followed by melt extrusion of the resulting polymer. The patent reports ABS as having up to 30% rubber.
U.S. Pat. Nos. 3,607,983 to Rushton et al., issued Sept. 21, 1971, and 4,042,647 to Cornell, issued Aug. 16, 1977, are similar in that they attempt to increase the rubber content of ABS by polymerizing styrene and acrylonitrile monomers onto a styrene-butadiene rubber of very small particle size. In the '983 patent, it is reported that the ABS has up to 50% rubber and that the particle size of the styrene-butadiene rubber particles is from 0.05 to 2 microns. In the '647 patent, an ABS is reported having up to 45% rubber content wherein 85% of the rubber particles are said to be less than 0.14 microns.
While artisans have recognized the desirability of increasing the rubber content in rubber reinforced polymer compositions such as ABS, the above references illustrate the difficulties encountered.